The present invention generally relates to a liquid crystal display, and more particularly to a liquid crystal display capable of preventing color shift and simultaneously improving its aperture ratio and transmittance.
A liquid crystal display device has been used in various information display terminals. The major operating system for the liquid crystal display device is the twisted nematic (xe2x80x9cTNxe2x80x9d) and the super twisted nematic (xe2x80x9cSTNxe2x80x9d). Though they are presently commercially used in the market, the problems of narrow viewing angle are still remained unsolved.
An In-Plane Switching (xe2x80x9cIPSxe2x80x9d) mode liquid crystal display has been suggested to solve foregoing problems.
As described in FIG. 1, a plurality of gate bus lines 11 are formed on a lower insulating substrate 10 along an x direction shown in the drawings and the gate bus lines 11 are parallel to each other. A plurality of data bus lines 15 are formed along an y direction which is substantially perpendicular to the x direction. Therefore a sub-pixel region is defined. At this time, a pair of gate bus lines 11 and a pair of data bus lines 15 are formed for defining the sub-pixel region. The gate bus line 11 and the data bus line 15 are insulated by a gate insulating layer(not shown).
A counter electrode 12 is formed, for example in a rectangular frame shape, within a sub-pixel region and it is disposed at the same plane with the gate bus line 11.
A pixel electrode 14 is formed at each sub-pixel region where the counter electrode 12 is formed. The pixel electrode 14 is composed of a web region 14a which divides the region surrounded by the rectangular frame type counter electrode 12 with a y direction, a first flange region 14b connected to a portion of the web region 14a and simultaneously overlapped with the counter electrode 12 of the x direction, and a second flange region 14c which is parallel to the first flange region 14c and is connected to the other portion of the web region 14a. That is to say, the pixel electrode 14 seems to be the letter xe2x80x9cIxe2x80x9d. Herein, the counter electrode 12 and the pixel electrode 14 are made of opaque metal layers. To ensure an appropriate intensity of electric field, the width of both counter and pixel electrodes is preferably 10-20 xcexcm.
The pixel electrode 14 and the counter electrode 12 are insulated from each other by a gate insulating layer (not shown).
A thin film transistor 16 (xe2x80x9cTFTxe2x80x9d) is disposed at the intersection of the gate bus line 11 and the data bus line 12. This TFT 16 is composed of a gate electrode being extended from the gate bus line 11, a drain electrode being extended from the data bus line 15, a source electrode being extended from the pixel electrode 14 and a channel layer 17 formed on upper of the gate electrode.
A storage capacitor Cst is disposed at the region where the counter electrode 12 and the pixel electrode 14 are overlapped.
Although not shown in FIG. 1, an upper substrate (not shown) equipped with a color filter (not shown) and a lower substrate 10 are disposed opposite to each other with a predetermined distance. Herein, the distance between the upper substrate and lower substrate is smaller than that between a region of the counter electrode toward the y direction and the web region of the pixel electrode thereby forming a parallel field which is parallel with the substrate surface. Further a liquid crystal layer (not shown) having a plurality of liquid crystal molecules is interposed between the upper substrate (not shown) and the lower substrate 10.
Also, onto the resultant structure of the lower substrate and onto an inner surface of the upper substrate are formed homogeneous alignment layers respectively. By the homogeneous alignment layer, before forming an electric field between the counter electrode 12 and the pixel electrode 14, long axes of liquid crystal molecules 19 are arranged parallel to the surface of the substrate 10. Also, by the rubbing axis of the homogeneous alignment layer, the orientation direction of the molecules 19 is decided. The R direction in the drawings is the direction of rubbing axis for the homogeneous alignment layer formed on the lower substrate 10.
A first polarizing plate (not shown) is formed on the outer surface of the lower substrate 10 and a second polarizing plate (not shown) is formed on the outer surface of the upper substrate (not shown). Herein, the first polarizing plate is disposed to make its polarizing axis to be parallel to the P direction of the FIG. 1. That means, the rubbing axis direction R and the polarizing axis direction P are parallel each other. On the other hand, the polarizing axis of the second polarizing plate is substantially perpendicular to that of the first polarizing plate.
When a scanning signal is applied to the gate bus line 11 and a display signal is applied to the data bus line 15, the TFT 16 disposed at the intersection of the gate bus line 11 and the data bus line 15 is turned on. Then the display signal of the data bus line 15 is transmitted to the pixel electrode 14 through the TFT 16. Consequently, an electric field E is generated between the counter electrode 12 where a common signal is inputted and the pixel electrode 14. At this time, the direction of electric field E is referenced as to x direction as described in the FIG. 1, it has a selected degree of angle with the rubbing axis.
Afterwards, before the electric field is not generated, the long axes of the liquid crystal molecules are arranged parallel to the substrate surface and parallel to the rubbing direction R. Therefore the light passed through the first polarizing plate and the liquid crystal layer is unable to pass the second polarizing plate, the screen has dark state.
When the electric field is generated, the long axes (or optical axes) are rearranged parallel to the electric field, and therefore the incident light passed through the first polarizing plate and the liquid crystal layer passes the second polarizing plate and the screen has white state.
At that time, the direction of the long axes of the liquid crystal molecules changes according to the presence of the electric field, and the liquid crystal molecules are arranged parallel to the substrate surface. Accordingly, a viewer can see the long axes of liquid crystal molecules at all points in the screen, and the viewing angle characteristic is improved.
However, the IPS mode liquid crystal display as described above also includes following problems.
As well known, the refractive anisotropy (or birefringence, xcex94n) is occurred due to the difference in the lengths of the long and the short axes of the liquid crystal molecules. The refractive anisotropy xcex94n is also varied from the viewer""s viewing directions. Therefore a predetermined color is appeared on the region where the polar angle is of 0 degree and the azimuth angle range of degrees 0, 90, 180 and 270 in spite of the white state. This regards as color shift and more detailed description thereof is attached with reference to the equation 1.
T≈T0 sin2(2"khgr")xc2x7sin2(xcfx80xc2x7xcex94nd/xcex) xe2x80x83xe2x80x83equation 1 
wherein,
T: transmittance;
T0: transmittance to the reference light;
"khgr": angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate;
xcex94n: birefringence;
d: distance or gap between the upper and lower substrates (thickness of the liquid crystal layer); and
xcex: wavelength of the incident light.
So as to obtain the maximum transmittance T, the "khgr" should be xcfx80/4 or the xcex94nd/xcex should be xcfx80/2 according to the equation 1. As the xcex94nd varies with the birefringence difference of the liquid crystal molecules depending on viewing directions, the value of xcex is varied in order to make xcex94nd/xcex to be xcfx80/2. According to this condition, the color corresponding to the varied wavelength xcex appears.
Accordingly, as the value of xcex94n relatively decreases at the viewing directions xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases. Consequently a blue color having shorter wavelength than a white color can be shown.
On the other hand, as the value of xcex94n relatively increases at the viewing directions xe2x80x9cbxe2x80x9d and xe2x80x9cdxe2x80x9d toward the short axes of the liquid crystal molecules, the wavelength of incident light relatively increases. Consequently a yellow color having a longer wavelength than the white color can be shown.
Deterioration is caused in the resolution of IPS mode liquid crystal displays.
There has been proposed a method employing many times of rubbing processes for preventing the color shift. However, the many times of rubbing processes incur troublesome processes, such as many times of photolithography and the alignment layer may be damaged.
It is one object of the present invention to provide a liquid crystal display capable of improving its picture quality.
It is another object of the present invention to provide a liquid crystal display having a multi-domain which is capable of manufacturing by a simplified process, excluding many times of rubbing processes.
So as to accomplish the objects, the present invention provides a liquid crystal display comprising:
a lower substrate having a plurality of gate bus lines being parallel each other and data bus lines being parallel each other to define pixels together with the gate bus lines, a thin film transistor provided at each intersection of the data bus line and the gate bus line, a pixel electrode formed in the pixel and connected to the thin film transistor, and a counter electrode formed in the pixel and forming electric field together with the pixel electrode;
an upper substrate opposed to the lower substrate to be separated apart;
a liquid crystal layer interposed between the lower substrate and the upper substrate, the liquid crystal layer including a plurality of liquid crystal molecules; and
homeotropic alignment layers formed between the lower substrate and the liquid crystal layer, between the liquid crystal layer and the upper substrate respectively;
wherein, the pixel is divided into a plurality of electric field-formed spaces by the counter electrode and the pixel electrode, and the electric fields formed in the respective spaces are formed as diagonal lines with respect to the gate bus lines and the data bus lines thereby making symmetries with those electric fields formed at adjacent spaces.
Further, the present invention provides a liquid crystal display comprising:
a lower substrate having a plurality of gate bus lines being parallel each other and data bus lines being parallel each other to define pixels together with the gate bus lines, a thin film transistor provided at each intersection of the data bus line and the gate bus line, a pixel electrode formed in the pixel and connected to the thin film transistor, and a counter electrode formed in the pixel and forming electric field together with the pixel electrode;
an upper substrate opposed to the lower substrate to be separated apart;
a liquid crystal layer interposed between the lower substrate and the upper substrate, the liquid crystal layer including a plurality of liquid crystal molecules; and
homeotropic alignment layers formed between the lower substrate and the liquid crystal layer, between the liquid crystal layer and the upper substrate respectively;
wherein, the counter electrode comprises: a first electrode of a rectangular frame shape; and at least a second electrode being parallel to the gate bus lines and dividing a space surrounded by the first electrode,
wherein the pixel electrode comprises: a first branch dividing the space surrounded by the first electrode in a direction crossing the second electrode; and a second branch crossing the first branch and interposed between a selected portion of the first electrode being parallel to the gate bus lines and the second electrode,
wherein the pixel is divided into a plurality of electric field-formed spaces by the counter electrode and the pixel electrode, the electric fields formed in the respective spaces are formed as diagonal lines with respect to the gate bus lines and the data bus lines thereby making symmetries with those electric fields formed at adjacent spaces.
The present invention also provides a liquid crystal display comprising:
a lower substrate having a plurality of gate bus lines being parallel each other and data bus lines being parallel each other to define pixels together with the gate bus lines, a thin film transistor provided at each intersection of the data bus line and the gate bus line, a pixel electrode formed in the pixel and connected to the thin film transistor, and a counter electrode formed in the pixel and forming electric field together with the pixel electrode;
an upper substrate opposed to the lower substrate to be separated apart;
a liquid crystal layer interposed between the lower substrate and the upper substrate, the liquid crystal layer including a plurality of liquid crystal molecules;
homeotropic alignment layers formed between the lower substrate and the liquid crystal layer, between the liquid crystal layer and the upper substrate respectively;
a polarizer disposed at an outer surface of the lower substrate wherein its polarizing axis is parallel to the gate bus lines or the data bus lines;
an analyzer disposed at an outer surface of the upper substrate wherein its absorbing axis is crossed with the polarizing axis; and
a phase compensating film having negative refractive anisotropy interposed between the upper substrate and the analyzer,
wherein, the counter electrode comprises: a first electrode of a rectangular frame shape; and at least a second electrode being parallel to the gate bus lines and dividing a space surrounded by the first electrode,
wherein the pixel electrode comprises: a first branch dividing the space surrounded by the first electrode in a direction crossing the second electrode; and a second branch crossing the first branch and interposed between a selected portion of the first electrode being parallel to the gate bus lines and the second electrode,
wherein the pixel is divided into a plurality of electric field-formed spaces by the counter electrode and the pixel electrode, the electric fields formed in the respective spaces are formed as diagonal lines with respect to the gate bus lines and the data bus lines thereby making symmetries with those electric fields formed at adjacent spaces.
In another aspect, the present invention provides a method of manufacturing liquid crystal display comprising the steps of:
depositing a metal layer on a lower substrate;
forming a counter electrode by patterning a selected portion of the metal layer, the counter electrode comprising a gate bus line extended in an x direction, a first electrode of a rectangular frame shape, a second electrode disposed in the x direction and dividing a space surrounded by the first electrode;
forming a gate insulating layer on the lower substrate in which the gate bus line and the counter electrode are formed;
depositing another metal layer on the gate insulating layer;
forming a pixel electrode by patterning a selected portion of the metal layer, the pixel electrode comprising a data bus line crossing the gate bus line, a first branch dividing the space surrounded by the first electrode in a direction crossing the second electrode of the counter electrode, a second branch crossing the first branch and interposed between a portion of the first electrode being parallel to the gate bus line and the second electrode;
forming a homeotropic alignment layer on the upper substrate;
preparing an upper substrate to be attached to the lower substrate;
forming a homeotropic alignment layer on the upper substrate structure;
attaching the lower substrate and the upper substrate so that the homeotropic alignment layers are opposed to be spaced apart; and
injecting liquid crystal between the lower substrate and the upper substrate.
In the presence of electric field, without performing the rubbing step at the alignment layers, there is formed a multi-domain within the pixel since at least one electric field in the diagonal shape is formed within the pixel according to the polarizing axis and the absorbing axis.
Accordingly, damages raised by rubbing the alignment layers are prevented since the present invention excludes the rubbing steps, thereby simplifying the manufacturing process. Further, there is formed a multi-domain and the color shift is not occurred.
Moreover, the polarizing axis of the polarizer is arranged in the directions of 90xc2x0, 180xc2x0 where the user mainly observes, therefore the contrast in the directions of 90xc2x0, 180xc2x0 can be remarkably improved.